Existing microelectronics technology can produce high-speed integrated circuits (IC's) that are capable of operating at frequencies well over one GHz. However, major difficulties have been encountered in interconnecting such high-speed IC's, and in packing them efficiently into compact multilayered boards and multichip modules. These difficulties have occurred in both analog and digital IC applications.
Conventional electrical interconnects comprising metallic conductors exhibit increasingly severe performance limitations when used for connecting high-speed IC's as operating frequencies become higher. Metallic conductors are more lossy at higher frequencies, thereby forcing IC network designers to use wider and shorter electrical interconnects that result in reduced packing and routing densities. Also, conventional electrical interconnects comprising metallic conductors exhibit significant crosstalk at high operating frequencies. (Each electrical interconnect in a high-speed IC network acts as a small antenna that broadcasts its information to neighboring interconnects and receives broadcasts from neighboring interconnects to produce crosstalk.) High power dissipation and high noise associated with conventional electrical interconnects at frequencies above a few hundred MHz have severely limited the usefulness of high-speed IC's in many applications.
The use of optical interconnects between high-speed IC's has been proposed and demonstrated in the prior art in attempts to avoid the problems associated with electrical (i.e., metallic) interconnects. With an optical interconnect, a time-varying electrical signal from an output pin on a transmitting IC can be used to directly modulate the optical output of a laser diode, or alternatively can be applied to an external modulator fed from a laser diode. Either of these techniques converts the electrical signal from the transmitting IC into a corresponding optical signal, which is conveyed from the laser diode or the external modulator to an optical receiver by a suitable optical transmission means.
The optical transmission means used for conveying the optical signal to the receiver can be an optical fiber (as typically used in long-haul telecommunications applications) or an optical waveguide (as typically used in interconnection board applications). The optical receiver can be a photoelectric detector such as an amorphous silicon photodetector or a photodiode, which converts the optical signal back into an electrical signal that is conducted to an input pin on a receiving IC. Thus, instead of using an electrical conductor to connect the output pin of the transmitting IC to the input pin of the receiving IC, an optical interconnect uses an optical transmission means in combination with means for performing electrical-to-optical conversion of the output of the transmitting IC and optical-to-electrical conversion of the input to the receiving IC.
The benefits of using optical signals instead of electrical signals to interconnect high-speed IC's include lower noise, lower propagation delay, higher packing density at high frequencies, and lower power dissipation, all of which significantly improve transmission capability. Optical signals transmitted by fibers or waveguides can carry information at very wide bandwidths, and are subject to considerably lower loss, power dissipation and crosstalk than are electrical signals transmitted by metallic conductors. However, these benefits can be realized to practical advantage, only if they offset the difficulties encountered in converting information from electrical to optical form at the output pin of the transmitting IC, and from optical to electrical form at the input pin of the receiving IC. In the prior art, difficulties in effecting electrical-to-optical and optical-to-electrical signal conversion have been major impediments to satisfactory implementation of optical interconnects in IC networks.
The primary difficulty in effecting electrical-to-optical and optical-to-electrical signal conversion in an IC network has been the requirement for an active function in the IC network to accomplish such conversion. The solution most commonly proposed in the prior art for achieving electrical-to-optical and optical-to-electrical signal conversion in an IC network has been to place an individual laser diode (either a discrete device or a monolithically integrated device) in the output line connected to each IC output pin. However, in a typical interconnection board or module having several hundred to several thousand IC output lines, the task of assembling discrete laser diode components is so complex that the cost of fabrication becomes prohibitive. Monolithic integration of laser diodes onto high-speed, high-performance IC's is a complex and difficult process that degrades the performance of both the laser diode transmitter and the IC itself. Furthermore, the area on an IC chip required for integrated laser diodes could in most cases be more effectively used for logic gates.
Other applications for which optical interconnects have been proposed include optical signal distribution networks, phased-array radars, optical backplanes, board-to-board connectors, crossbar and other types of switching networks, and, in general, all types of fiber-optic systems and integrated optical systems. However, in all such applications, problems as described above have been encountered or anticipated in attempting to implement optical interconnects. In summary, the problems inherent in the techniques of the prior art for implementing optical interconnects for IC networks are primarily as follows:
1) A separate laser diode is required for each optical interconnect. Thus, an IC network requiring thousands of interconnects would correspondingly require thousands of laser diodes.
2) The electrical signal at the output pin of each IC in the network is converted to an optical signal, either by modulating the output of a laser diode directly at high frequency or by using an external modulator to modulate a CW laser beam. Thus, for an IC network requiring thousands of interconnects, it would be necessary to accommodate the inefficiencies and operating instabilities inherent in modulating thousands of laser diodes at high frequency.
A need has been perceived in the prior art for a technique whereby IC electrical signals can be efficiently converted at high frequency into optical signals, and whereby the optical signals so produced can be efficiently a network.